02-22-2021, 09:39 PM
Hi Bob,
RadioRoslyn has it right. I did not know that there was a name for it. This was a somewhat common way in the early and mid 1930s to wire the filter capacitors and choke (the speaker filed coil doubles as a choke. It was even done on the radiola 17, 18 and 60 (Ca 1927, 1928) It messed my head up a little when I first saw it, but do understand the reasons:
First, it may be for safety. Because the Speaker Field is on the low side, if it ever shorts, then the voltage to ground will be a lot less than the 250 - 350V if it was on the positive side of the supply.
Note also that the cathode of the 6F6 Audio Output tube is at chassis ground along with the negative lead of one of the filter capacitors.. the grid resistor is at the Power transformer center tap along with the negative lead of the other filter capacitor
The arrangement in the schematic provides negative bias without the need for a cathode resistor bypass capacitor. My Westinghouse WR217 has almost the same circuit. early RCAs such as my R7A Suprette also did the same thing. Hope this helps.
RadioRoslyn has it right. I did not know that there was a name for it. This was a somewhat common way in the early and mid 1930s to wire the filter capacitors and choke (the speaker filed coil doubles as a choke. It was even done on the radiola 17, 18 and 60 (Ca 1927, 1928) It messed my head up a little when I first saw it, but do understand the reasons:
First, it may be for safety. Because the Speaker Field is on the low side, if it ever shorts, then the voltage to ground will be a lot less than the 250 - 350V if it was on the positive side of the supply.
Note also that the cathode of the 6F6 Audio Output tube is at chassis ground along with the negative lead of one of the filter capacitors.. the grid resistor is at the Power transformer center tap along with the negative lead of the other filter capacitor
The arrangement in the schematic provides negative bias without the need for a cathode resistor bypass capacitor. My Westinghouse WR217 has almost the same circuit. early RCAs such as my R7A Suprette also did the same thing. Hope this helps.
"Do Justly, love Mercy and walk humbly with your God"- Micah 6:8
Best Regards,
MrFixr55
